Permanent magnet compositions based on the rare earth (RE) elements neodymium or praseodymium or both, the transition metal iron or mixtures of iron and cobalt, and boron are known. Preferred compositions contain a large proportion of an RE.sub.2 TM.sub.14 B phase where TM is one or more transition metal elements including iron. A preferred method of processing such alloys involves rapidly solidifying molten alloy to achieve a substantially amorphous to very finely crystalline microstructure that has isotropic, permanently magnetic properties. In another preferred method, overquenched alloys without appreciable coercivity can be annealed at suitable temperatures to cause grain growth and thereby induce magnetic coercivity in a material having isotropic permanently magnetic properties.
It is also known that particles of rapidly solidified RE-Fe-B based isotropic alloys can be hot pressed into a substantially fully densified body and that such body can be further hot worked and plastically deformed to make an excellent anisotropic permanent magnet. Thus, alloys with overquenched, substantially amorphous microstructures are worked and plastically deformed at elevated temperatures to cause grain growth and crystallite orientation which result in substantially higher energy products than in the best as-rapidly-solidified alloys. The maximum energy product to date for hot worked, melt-spun Nd-Fe-B magnet bodies is up to about 50 MGOe, although energy product as high as 64 MGOe are theoretically possible.
As stated above, the preferred rare earth (RE)-transition metal (TM)-boron (B) permanent magnet composition consists predominantly of RE.sub.2 TM.sub.14 B grains with an RE-containing minor phase(s) present as a layer at the grain boundaries. It is particularly preferred that on the average the RE.sub.2 TM.sub.14 B grains be no greater than about 500 nm in greatest dimension in the permanent magnet product.
While such hot die upsetting is suitable for its intended purpose, in certain manufacturing processes it would be desirable to directly convert the isotropic particles to anisotropic permanently magnetic particles. Such anisotropic particles can then be mixed with a suitable matrix material and shaped to form a bonded permanent magnet having magnetically anisotropic properties.